Closed loop current sensors interface with a high magnetic permeability core encircling a primary winding or primary conductor, with a secondary winding or compensation coil driven by the sensor until the magnetic flux within the core is zero. The secondary current at this point is measured and is generally proportional to the primary current. Closed loop current sensors provide good accuracy and galvanic isolation and are thus preferred solutions for sensing current flow in many industrial applications. In operation, the core of the closed loop current sensor operates like a transformer core with respect to the primary and secondary currents. However, the core may be magnetized over time, leading to an offset and degradation in the accuracy of the current sensing performance. For example, exposure of the core to strong fields from external magnets or overcurrent conditions in a host system may lead to undesirable magnetization of the transformer core. Furthermore, the core may be magnetized through significant primary current flow while the sensor is unpowered. Magnetization of the transformer core leads to a magnetic domain offset, causing an offset in the feedback current applied to the secondary winding, and thus limits the precision and dynamic range of the magnetic current sensor. Moreover, it is difficult to track or predict the amount of such magnetization for different cores.
Sensor core magnetization has been addressed in part through calibration or offset cancellation techniques in the electrical domain. However, this approach does not reduce the magnetic domain offset. In addition, offset calibration in the electrical domain is expensive and time-consuming, and susceptible to thermal drift.
Open loop degaussing or demagnetizing techniques have been tried, in which an AC signal is applied to the magnetic core that increases in frequency and/or decays in amplitude. These degaussing techniques, however, cannot reliably achieve the accuracy required for many applications, such as differential current sensing in which the (small) difference between two primary currents must be sensed accurately. In particular, timing errors, external magnetization effects during the degaussing operation, and the uncertainty in the end position on the flux density-magnetic field strength (B-H) curve when the demagnetization process completes limit the ability to accurately degauss a sensing core, and these techniques commonly only achieve a final accuracy of ±10% of initial magnetization. Furthermore, it is not possible to record the initial state of magnetization to keep track of systematic magnetization. Another drawback is the length of time required for degaussing, since lack of knowledge regarding the initial magnetization level requires full magnetization of the core in a certain direction, and then a full-length demagnetization sequence in the other direction. Thus, while existing degaussing and offset calibration options provide some improvement over operating with a magnetized transformer core, many applications for close loop magnetic current sensors require accuracies that cannot be achieved using these techniques. A need therefore remains for improved degaussing or demagnetizing methods and apparatus for closed loop magnetic current sensors and other demagnetization applications.